1. Field of the Invention
This invention relates an ion conducting membrane having a matrix including an ordered array of hollow channels and a nanocrystalline electrolyte contained within at least some of the channels. The channels have opposed open ends, and a channel width of 1000 nanometers or less, and most preferably, the channels have a width of 10 nanometers or less. The electrolyte has grain sizes of 100 nanometers or less, and preferably the electrolyte has grain sizes of 1 to 50 nanometers. In one form, the electrolyte conducts oxygen ions, the matrix is silica, and the electrolyte is yttrium doped zirconia.
2. Description of the Related Art
Oxide-based ionic conductors, such as oxygen-conducting or proton-conducting ceramic membranes, are extremely important materials for a wide-range applications such as in fuel cells (electrolytes or electrodes), sensors, gas separation and catalysis, microbatteries, thermoelectric and magnato hydrodynamic generators, and other solid-state ionics-based devices. (See, for example, U.S. Pat. Nos. 4,933,054, 5,827,620, 6,004,688, 6,153,163, 6,207,311, 6,558,831, 6,562,747, 6,653,009, and 6,677,070 which are incorporated herein by reference along with all other publications cited herein.) Several recent review articles have well documented the current development of various ionic or mixed conducting oxides, including stabilized zirconia, ceria, and lanthanide oxides (see, for example, Goodenough, “Oxide-ion electrolytes,” Annu. Rev. Mater. Res., 33, 91, 2003; Adachi et al., “Ionic conducting lanthanide oxides,” Chem. Rev., 102, 2405, 2002; and Kreuer, “Proton-conducting oxides,” Annu. Rev. Mater. Res., 33, 333, 2003). A major technological challenge is regarding how to create and utilize these potential “candidate” conducting oxides in the form of thin-layer membranes (see, De Jonghe et al., “Supported electrolyte thin film synthesis of solid oxide fuel cells,” Annu. Rev. Mater. Res., 33, 169, 2003; and Will et al., “Fabrication of thin electrolytes for second-generation solid oxide fuel cells,” Solid State Ionics, 131, 79, 2000) that promise much higher ionic conductivity across the membrane than any existing ceramic membrane can offer.
It is believed that fuel cells—as cleaner and more efficient energy conversion systems, will reshape the future of automotive propulsion, distributed power generation, and low power portable devices (battery replacement) (see Carrette et al., “Fuel cells: principles, types, fuels, and applications,” CHEMPHYSCHEM, 1, 162, 2000). Solid oxide fuel cells present an efficient and ecologically acceptable way to simultaneously generate heat/electricity with theoretical density as high as 70% and with low emissions (see Oljaca et al., “Nanomaterials for solid oxide fuel cells,” American Ceramic Society Bulletin, 82 (1), 38, 2003). In a typical solid oxide fuel cell, the oxygen reduction reaction taking place at the cathode is: O2+4e−→2O2−. The O2− ion is transferred from the cathode through the electrolyte to the anode. One oxidation reaction taking place at the anode is: 2H2+2O2−→2H2O+4e−. The oxidation reaction at the anode, which liberates electrons, in combination with the reduction reaction at the cathode, which consumes electrons, results in a useful electrical voltage and current through the electrical load.
Widespread commercialization of solid oxide fuel cells is limited by their high operating temperature (>800° C.) and thus high system cost. One future development goal for intermediate-temperature solid oxide fuel cells is to introduce more promising alternative materials (such as electrolyte and electrode materials) that would enable lowering the operating temperature from 1000° C. to below 800° C. without loss of performance (see McEvoy, “Thin SOFC electrolytes and their interfaces—a near-term research strategy,” Solid State Ionics, 132, 159, 2000; Huijsmans, “Ceramics in solid oxide fuel cells,” Current Opinion in Solid State and Materials Science, 5, 317 2001; De Jonghe et al. supra; and Brandon et al., “Recent advances in materials for fuel cells,” Annu. Rev. Mater. Res., 33, 183, 2003). Lower temperature operation or processing will reduce the system materials requirement and cost, and also avoid many undesirable interfacial reactions (e.g., formation of insulating interphase of lanthanum zirconate) between electrode and electrolyte materials (see McEvoy supra). At low operation temperatures, superior high ionic conductivity of the oxide electrolyte layer (>10−2 S/cm) is required for the success of future fuel cell technology. Thus, there is a need for a stable electrolyte membrane which offers higher ionic conductivity at lower temperatures.
Ionic conductivity in solid electrolytes can be improved by dissolving appropriate impurities/dopants into the structure or by introducing interfaces (such as grain boundaries) that cause the redistribution of ions in the space-charge regions (see, Sata et al., “Mesoscopic fast ion conduction in nanometer-scale planar heterostructures,” Nature, 408, 946, 2000). However, doping has its limitations in enhancing conductivity.
Nanomaterials with the control of microstructure down to the nanoscale have been considered promising in improving the materials performance for solid-state ionics and solid oxide fuel cells (see Tuller, “Defect engineering: design tools for solid state electrochemical devices,” Electrochimica Acta, 48 (20-22), 2879, 2003; and Oljaca et al, supra). It has been reported that conductivity in the nanocrystalline grain-boundary regions is greater than for larger grains, due to fast ion diffusion through grain boundaries (see Tuller, “Ionic conduction in nanocrystalline materials,” Solid State Ionics, 131,143, 2000). For example, nanocrystalline solid oxide electrolytes (such as yttrium stabilized zirconia and cation-doped CeO2) have typically shown orders-of-magnitude higher conductivity than those in microcrystalline oxide ceramics (see Kosacki et al., “Nonstoichiometry and electrical transport in Sc-doped zirconia,” Solid State Ionics, 152-153, 431, 2002; Kosacki and Anderson, “Grain boundary effects in nanocrystalline mixed conducting films,” Encyclopedia of Materials: Science and Technology, Elsevier Science Ltd., pp. 3609-3617, 2001; Suzuki et al., “Microstructure-electrical conductivity relationships in nanocrystalline ceria thin films,” Solid State Ionics, 151, 111, 2002; Suzuki et al., “Defect and mixed conductivity in nanocrystalline doped cerium oxide,” J. Am. Ceram. Soc., 85, 1492, 2002; and Suzuki et al., “Electrical conductivity and lattice defects in nanocrystalline cerium oxide thin films,” J. Am. Ceram. Soc., 84, 2007, 2001). A few methods, including nanopowder consolidation, sol-gel, and polymer precursor coating, have been investigated for making nanocrystalline phase electrolyte films/membranes (see, Dong and Hu et al., “Grain growth in nanocrystalline yttrium-stabilized zirconia thin films synthesized by spin coating of polymeric precursors,” J. Nanosci. Nanotechnol., 2, p. 161-169, 2002; Menzler et al., “Materials synthesis and characterization of 8YSZ nanomaterials for the fabrication of electrolyte membranes in solid oxide fuel cells,” Ceramics International, 29, 619, 2003; and Zhu, “Fast ionic conducting film ceramic membranes with advanced applications,” Solid State Ionics, 119, 305, 1999). However, none of these membranes are suitable for providing enhanced cross-membrane conductivity. Furthermore, the intrinsic problem of nanograin growth (i.e. thermal stability of nanostructure) in nanocrystalline films/membranes at high temperatures reduces the conductivity. Therefore, there is also a need for strategies to maintain the small nanocrystal grain size and to fully utilize grain boundary interfaces in order to enhance and maintain ionic conductivity.